1. Field of the Invention
The present invention relates to energy systems as used on drilling rigs. More particularly, the present invention the relates to drilling rigs that are supplied with power from a dual fuel engine/generator. Additionally, the present invention relates systems for supplying power and for storing power through the use of batteries and other energy storage systems.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
In the field of oil well drilling, a significant amount of power is required during the drilling activity. The power requirements, as used on a drilling rig, serve to supply the drawworks, the mud pumps, the top drives, the rotary tables, the dynamic braking systems and other peripheral loads. In oil well drilling activities, oversized power systems are often utilized so as to meet the “peak” power requirements.
Historically, the number of engines/generators that are used and are typically online are more than the required load of the application due to the redundancy and necessary peak KW and VAR demand during certain aspects of the operation. In particular, these peak demands are during the “tripping” of the pipe or drill stem.
During normal operations, there is a base load of lighting, pumps, agitators, mixers, air compressors, etc. This base load can make up typical loads of 400-600 kilowatts. The mud pumps, top drives and rotary tables contribute another fairly consistent KW demand. This demand will vary based on the particular well, depth of drilling, and material being drilled.
During oil well drilling activities, the most intermittent load is the drawworks. This intermittent load is directed toward the peak demand during the raising or lowering of the drill pipe upwardly and downwardly in the well. This peak demand can have loads as much as two to three times the base loads of the other demands on the drilling rig.
When drilling and at times when the downhole tool has to be inspected or changed, it is required to pull all of the drill pipe from the hole. This distance can be 10,000 feet or more. The drill pipe must be taken apart and stacked as it is being removed. After repair or replacement, the reverse procedure must take place so as to reinsert all the components back to the desired depth. During the tripping in or out of the hole, the driller (operator) demands extreme power consumption and very quick bursts as the driller raises (or lowers) the string of drill pipe. Since there is a limitation on the height of the drilling mast, the operator must lift the sections in increments and unscrew the different sections. These sections are stacked one at a time. This process is repeated during the reinsertion of the drill pipe back into the hole. This process is referred to as “making a trip”. The intermittent high demand occurs when this load (300,000 pounds or more) occurs over and over again. The load is inconsistent since the weight of the drill stem becomes less and less as sections are removed. The base load requirements for the drilling rig are approximately 600 to 800 KW. The peak demand can be 1.5 MW and as high as 2.0 MW. Because of these power requirements, the emissions of the engines/generators for a typical land rig are quite high. Newer engines can have much lower NOx output than earlier engines. There are also large amounts of carbon dioxide emissions. The fuel consumption during these intermittent demands can be quite significant.
On mechanical rigs, power from the engines drives the rig equipment either directly, through a clutch, or through a torque converter. Electric rigs use engine power to drive one or more generators. The generated electricity is then used to operate motors for the larger equipment on the rig. There are three types of electric rigs, direct current, silicon-controlled rectifiers, and variable frequency drives. Direct current rigs have a DC generator that supplies power to DC motors. These are the oldest types of drive systems. The silicon-controlled rectifier systems produce AC power from the generators and then changed to DC by switchgear in order to power DC motors. This allows for more power to be generated by smaller generators. Variable frequency drives are the newest kind of rig which utilize variable speed AC motors so as to allow for even more power output for the same sized equipment.
There are various ignition methods that are used in the reciprocating internal compression engines used as the generator for electric drilling rigs. These ignition methods include compression ignition and spark ignition. Diesel engines are one type of compression ignition engine. Combustion air is first compression heated in the cylinder and diesel fuel oil is then injected into the hot air. Ignition is spontaneous because the air temperature is above the autoignition temperature of the fuel. Spark ignition initiates combustion by the spark of an electrical discharge. This engine is a dedicated natural gas engine and offers the greatest fuel cost savings and emission reductions in comparison to diesel engines.
Although all diesel-fueled engines are compression-ignited and all gas-fueled engines are spark-ignited, natural gas can be used in a compression ignition engine if a small amount of diesel fuel is injected into the compressed natural gas/air mixture so as to burn any mixture ratio of natural gas and diesel oil. This type of engine is often referred to as a “dual fuel” engine. Compression ignition engines usually operate at a higher compression ratio (ratio of cylinder volume when the piston is at the bottom of its stroke to the volume when it is at the top) than spark ignition engines because fuel is not present during compression. Hence there is no danger of premature autoignition. Since engine thermal efficiency rises with increasing pressure ratio (and pressure ratio varies directly with compression ratio), compression ignition engines are more efficient than spark ignition engines. This increased efficiency is gained at the expense of poorer response to load changes and the need for a heavier structure to withstand the higher pressures.
Natural gas generators are being used for land-based drilling applications and offer unique advantages in reduced exhaust emissions and significant fuel cost savings compared to more commonly-used diesel engine generators. Natural gas engine generators make it simpler to meet ever more stringent emissions regulations, particularly for oxides of nitrogen (NOx). Additionally, natural gas engine generators have the added advantage of accepting wellhead gas for further cost benefits. Diesel engines have much better load characteristics when compared to natural gas engines and therefore respond more reliably to changes in loads as drilling functions abruptly demand power requirements, such as tripping of the drill string.
The dual fuel engine is a compression ignition engine that operates on gaseous fuels while maintaining some liquid fuel injection to provide a deliberate source for ignition. Such a system is usually designed to minimize use of diesel fuel by replacing it with various gaseous fuels and their mixtures while maintaining satisfactory engine performance. Dual fuel engines offer reduced fuel costs and emissions benefits compared to conventional diesels. However, this benefit can be limited since the generator must occasionally switch from higher volume ratios of natural gas back to higher volume ratios of diesel fuel to meet the block loading and load-shedding conditions forced by changing rig power demands.
There are several approaches for dual fuel engine technologies. One approach is a dedicated dual fuel design which uses a direct metered cylinder charge of natural gas so as to permit natural gas levels approaching 99% with extremely low pilot fuel levels (near 1%). This is a purpose-built engine and consequently is very expensive. This engine is best suited for steadier, regulated loads. Typically, this type of engine is ill-suited for the dynamic loads of drilling rigs.
Another type of dual fuel engine/generator is the after market “fumigation system” adapted to almost any diesel engine brand. As such, each engine will have different displacements, diesel fuel injection systems, compression ratios, turbocharger boosts, intake manifold systems, cooling systems, and operate at different speeds. As such, the fuel ratio with a fumigation system will be inherently different on almost every engine. An important aspect of achieving optimum substitution with a fumigation system is reaching the “sweet spot” range of the particular engine by maintaining the ideal load.
There are problems associated with conversion of a conventional diesel engine to dual fuel operation. At light loads, dual fuel engines tend to exhibit inferior fuel utilization and power production efficiencies. There is higher unburned gaseous fuel and carbon monoxide emissions relative to corresponding diesel performance. Operation at light loads is also associated with greater cyclic variation in performance parameters, such as peak cylinder pressure, torque and ignition delay. This has narrowed the effective working range for dual fuel applications in the past. These trends arise mainly as a result of poor flame propagation characteristics within the very lean gaseous fuel/air mixtures and the origination of the various ignition centers of the pilot. The quality of natural gas used to fuel a converted engine, with respect to its percentage makeup of component gases, will directly affect power, efficiency, emissions, and longevity of the engine. In these fumigation-type dual fuel systems, there are several concerns relative to the natural gas/diesel ratio, the knock limit, and the maximum load rating. Gas composition, engine load factor, engine control strategy, engine condition, charge-air temperature and ambient conditions (temperature and altitude) govern the upper limit of gas substitution in most cases. Gas ratio is typically limited by the knock limit of the air-natural gas mixture at a particular engine load. In general terms, high quality gas and moderate engine loads (up to 70% of stand-by rating), will typically yield gas ratios between 30-70%. Lower quality natural gas, high engine loads, high charge-air temperatures and high altitude (or a combination of these factors) will typically limit gas ratio.
The gas substitution that is possible varies depending on gas quality, engine design, engine model and condition, engine load factor, charge air temperature (aftercooling), and ambient conditions (altitude and temperature), but should never exceed 70%, even under the most ideal conditions. In general, high quality gas (over 95% CH4), combined with moderate engine power levels and low temperature aftercooling will typically yield gas ratios in the 60% to 70% range, although there are factors that can still limit this value to much lower levels. Lower quality gas, combined with high manifold air temperature and/or higher engine loads, will typically result in gas substitution closer to 50% or lower.
In most applications, engine knock (detonation) will be the limiting factor in determining maximum gas ratio. In most cases, short duration knock will not cause harm to the engine. However, extended operation in a knocking condition may result in engine damage or failure. A knocking condition can be diagnosed both audibly and by using the bi-fuel system vibration sensor data. Data from the engine vibration sensors should be monitored closely during the setup procedure to confirm proper engine operation. If knocking is detected during bi-fuel operation, the engine should be rapidly switched to 100% diesel operation. To prevent recurrence of knocking, a reduction in gas ratio and/or a reduction in engine load will be required.
Engines that are converted to dual fuel operation are typically utilized for peak shaving, prime power, co-generation, or other high use applications. It is important for the installing technician to understand the power rating system used for most high-speed diesel engines, and the associated duty-cycles applicable to each. Most manufacturers of high speed (1200-1800 rpm) diesel engines and generator sets publish stand-by, prime and continuous ratings. The stand-by rating is reserved for emergency operation only and represents the highest horsepower or work level that can be sustained for a limited period of time. In most applications, the stand-by rating will not be used for bi-fuel operation. The prime rating typically allows for unlimited hours of use, with a variable load, up to the prime rated output. The continuous rating is the most conservative rating, and is reserved for unlimited hours at a constant load. In general, bi-fuel mode is reserved for operations at or below the prime rating of the machine. The higher the number of hours of intended use and the more constant the load rate, the more conservative the rating should be.
Referring to FIG. 1, there is shown a prior energy system for use with the various loads of a drilling rig. In particular, the energy system 10 includes engines 12, 14 and 16. Engine 12 operates generator 18. Engine 14 operates generator 20. Engine 16 operates generator 22. The generators 18, 20 and 22 will pass AC power along respective lines 24, 26 and 28 to a common AC bus 30. Typically, the various engine/generators, as shown in FIG. 1, are diesel engines. However, it is possible that such engine/generator combination could be also natural gas engine/generators.
A common DC bus 32 is illustrated as connected to the various components 34, 36, 38, 40 and 42 of the drilling rig. Load 34 is a DB module. Load 36 is the drawworks. Load 38 is the top drive. Loads 40 and 42 are the mud pumps. Each of these loads 34, 36, 38, 40 and 42 are switchably connected to the common DC bus 32. The AC bus is configured to supply power to the hotel loads 44 and 46 of the drilling rig. Hotel loads 44 and 46 can include air-conditioning and heating, lighting, and other energy requirements of the drilling rig. A first rectifier 48 is connected between the AC bus 30 and the DC bus 32. Rectifier 38 serves to convert the AC power to DC power. Similarly, the other rectifier 50 is connected between the AC bus 30 and the DC bus 32, also to convert the AC power to DC power. The DC power is properly utilized by the loads 34, 36, 38, 40 and 42. In FIG. 1, it can be seen that there is a resistive load bank 52 that is connected, by a switch, to the AC bus. As such, any excess energy that is provided by the various engine/generator combinations can be burned as heat by the resistive load bank 52.
Currently-used natural gas engine/generators that are used to power a drilling rig must be controlled to accept a lower level of transient response than is possible with diesel power. This requires the estimating of the transient response capability of the natural gas engine/generator and the determining of how the rate of application or rate of load removal can be reduced to make the system work. Unfortunately, this results in reduced power rates and decreased rig productivity, even with the use of a ballast load or the resistive load bank 52. A typical approach is to create a load profile of the rig's expected operations in terms of power required versus time. The creation of this profile for both the desired “ideal” loading rates and for the drill site's minimum requirements will establish the minimum and maximum loading conditions for the rig powerhouse. Gas engine/generator operation is then controlled within these minimum and maximum values to attempt to minimize power interruptions from forced generator failure.
FIG. 2 shows the transient response of the natural gas engine/generator during the adding of load or the shedding of load. All gensets have a response to such added load or shredded load. Changes in voltage and frequency associated with this transient response is dependent on the generator type (e.g., diesel compression versus natural gas spark-ignited engine) and the magnitude of the load change, where these step loads are described as some percentage of full rated power.
The transient response and steady state stability of generator set engines can vary because of a number of factors, such as engine model, engine speed, aspiration, power factor, governor and the presence of an idle circuit. Diesel engines have a short mechanical path between the governor actuator and the fuel delivery system to the combustion chamber. This system responds quickly and in a more stable manner to load change requests from the governor. Whenever a large load is added to a generator set, engine speed temporarily slows down, or dips, before returning to its steady-state condition. When a load is removed, engine speed increases, or overshoots, temporarily. Since generator frequency is determined by engine RPM, the quality of electrical power is impacted. The measurements of these temporary speed changes is referred to as “transient response”.
In the past, various patents and patent publications have been issued that relate to power usage and the control of such power usage by drilling rig systems. For example, U.S. Pat. No. 4,590,416, issued on May 20, 1986, to Porche et al., teaches a closed loop power factor control for power supply systems. This power factor controller for alternating current/direct current drilling rigs. The power factor controller utilizes a uniquely controlled, unloaded, over-excited generator to reactive power to maintain the rig's power factor within prescribed limits during peak demand operations. In particular, this method includes the step of: (1) sensing the instantaneous system power factors; (2) comparing the sensed instantaneous power factor to a prescribed power factor; (3) forming a power factor control signal indicative of the difference between the sensed power factor and the prescribed power factor; (4) providing a field excitation signal to an unloaded over-excited generator operated in the motor mode in proportion to the power factor control signal so as to cause the over-excited generator to generate the requisite reactive power to correct the system's power factor to the prescribed power factor; and (5) coupling the output of the over-excited generator to the power system.
U.S. Patent Publication No. 20088/0203734, published on Aug. 28, 2008 to Grimes et al., describes a wellbore rig generator engine power control system. This system controls power load to a rig engine. This system includes a sensor for controlling a rig engine and a sensor for sensing the exhaust temperature of a rig engine. The sensor is in communication with the controller so as so as to provide the controller with signals indicative of the exhaust temperature. The controller maintains power load to the rig engine based on the exhaust temperature.
U.S. Patent Publication No. 2009/0195074, published on Aug. 6, 2009 to Buiel, shows an energy supply and storage system for use in combination with a rig power supply system. This system includes a generator start/stop and a power output control. A bi-directional AC/DC converter converts the AC power generated by the engine-generator. The power supply is adapted to draw energy from the storage system when the rig motor exceeds the capacity of the generator.
U.S. Patent Publication No. 2009/0312885, published on Dec. 17, 2009 to Buiel, teaches a management system for drilling rig power supply and storage. This management system has a power generator coupled to rig loads. The power generator is used for powering and charging the storage system. The energy storage system draws energy from the storage system in periods of high power requirements and distributes excess energy to the storage system in periods of lower power requirements. The output of the power generator is managed based on the rig power usage wherein the output is increased when the rig power requirements are above a preselected threshold and wherein the output is decreased when the rig power requirements fall below a preselected threshold.
U.S. Patent Publication No. 2011/0074165, published on Mar. 31, 2011 to Grimes et al., describes a system for controlling power load to a rig engine of a wellbore rig. The system includes a controller for controlling the rig engine and a sensor for sensing the exhaust temperature of the rig engine. The sensor is in communication with the controller for providing to the controller signals indicative of the exhaust temperature. The controller maintains the power load to the rig engine based on the exhaust temperature.
U.S. Pat. No. 7,311,248, issued on Dec. 15, 2009 to the present inventor, provides a system for managing energy consumption in a heave-compensating drawworks. This system includes a power supply, a winch drum connected to the power supply so as to receive power from the power supply, a flywheel connected to the winch drum and to the power supply, and a controller connected to the power supply and to the winch drum for passing energy to and from the flywheel during an operation of the winch drum. The flywheel includes a disk rotatably coupled to an AC motor. This power supply includes a first pair of AC motors operatively connected on one side of the winch drum and a second pair of AC motors operatively connected on an opposite side of the winch drum.
It is an object of the present invention to provide an energy storage system for use on a drilling rig which allows a dual fuel engine/generator to operate with the same reliability and responsiveness as that of a diesel engine/generator.
It is another object of the present invention to provide an energy storage system which improves rig efficiency through energy recovery.
It is another object of the present invention to provide an energy storage system which serves to reduce the amount of wasted fuel that had previously been lost in resistive load banks.
It is another object of the present invention to provide an energy storage system which can reduce natural gas fuel consumption and reduce emissions.
It is a further object of the present invention to provide an energy storage system which allows operators to utilize wellhead gas as the fuel for the generator system.
It is still another object of the present invention to provide an energy storage system which serves as an uninterruptible power supply for use during fuel interruptions.
It is still a further object of the present invention to provide an energy storage system which reduces the vulnerability of the generator's output to variations in wellhead gas flow rates and methane contents.
It is still a further object of the present invention to provide an energy storage system which allows a dedicated or fumigation-type dual fuel engine/generator to be utilized in association with the production of power for a drilling rig.
It is still a further object of the present invention to provide an energy storage system which eliminates the requirement for auxiliary diesel engine/generator sets.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.